Dry ice – solid carbon dioxide – melts and becomes gas. Workers place the ice into the exits of rat burrows, where it breaks down over a period of hours and gasses the rats, who eventually die underground.

“There’s no doubt, it’s the best tool I’ve ever had for dealing with rodents,” said Stellberger, who used dry ice for four years for his private clients.

Stellberger and his team trained the city of Boston’s pest hunters on the process of using dry ice to combat a growing rat problem.

But half a year after it was introduced to Boston, dry ice for rodent control has been banned in the city. In late October, the Massachusetts Department of Agricultural Resources issued a cease and desist order to the city of Boston. Under the order, which was initiated by the U.S. Environmental Protection Agency, city officials must register dry ice as a pesticide before they use it to kill rodents.

Launching the movement

Boston became the first major U.S. city to use dry ice for rodent control, and advised New York and Chicago, which soon followed suit.

“It’s a no-brainer. I would encourage it certainly,” said Stellberger, whose team held a public demonstration of the technique in late April.

It is illegal to use dry ice as a rodenticide, under the Federal Insecticide, Fungicide, and Rodenticide Act, known as FIFRA, according to the EPA.

“If manufacturers represent the product as being effective at controlling rodents, or if they have knowledge that the product they are selling is going to be used as a rodenticide, then they are subject to FIFRA and would be in violation of selling and distributing an unregistered pesticide,” the EPA said in a statement.

“(C)ities, school districts or other persons responsible for such applications could be penalized under FIFRA,” the EPA said.

It's disappointing news for ISD Commissioner William “Buddy” Christopher at a time when he needs all the help he can get.

“We were a little taken aback when we got the notice from the EPA saying that because of a procedural requirement that it's not listed on one of their recognized poisons, we had to stop using it,” Christopher said.

5 Investigates and Northeastern University School of Journalism analyzed thousands of rodent complaints made to the city and discovered that 2016 is the most complained-about year on record. People made 3,524 complaints so far this year, 776 more than two years ago, a 28 percent increase.

The city attributes some of the increase in rodent complaints on the ability to easily report rodent sightings with the 311 app which could result in multiple complaints about the same incident.

Rat populations are up worldwide, according to Stellberger, who is bothered about the EPA ban.

“All the strides we've made forward for this green approach has taken a step back and it hurts,” he said.

Stellberger is also disturbed because he believes the objections to dry ice have more to do with pest control competitors who don't use the technique.

“Why do you think they're complaining?” Beaudet asked him.

“I guess we really have to ask them, but probably because we were taking business from them,” Stellberger replied.

Emails obtained by 5 Investigates show one pest control company wrote to state regulators in July that “it is irresponsible to use this approach without a ruling from the EPA."

The pest control company that contacted the state about dry ice tells us it did so because dry ice was being used illegally, without EPA approval, not because of concerns about losing business. The state tells us it had already been looking into the issue before getting any complaints.

Back on the front lines in Boston, the city is sticking to traditional methods for now, including baiting and trapping rodents. But the city is working to get the proper approvals from the EPA.

“We'll continue our efforts, using poison and the acceptable industry standards as we go forward. I would prefer to be using dry ice,” Christopher said.

This investigation was reported on for a seminar in investigative reporting taught by 5 Investigates’ Mike Beaudet who is also a journalism professor at Northeastern University. The following students participated in the project: Olivia Arnold, Alison Berstein, Audrey Cooney, Matthew MacCormack, Scott Shurtleff, Maxim Tamarov, and Zachary Tweed.